In a projection type projector apparatus, an image with sufficient color rendering property is required to be uniformly projected onto a rectangle screen. For this reason, a metal halide lamp, in which enclosing mercury or metal halide, is used as a light source. Moreover, in recent years, advancements in and miniaturization of a point light source has put lamps with electrodes that have a very short distance between into practical use. In these situations, a high pressure discharge lamp with very high mercury vapor pressure, for example, 20 MPa or more, has been recently used, instead of a metal halide lamp. This lamp is designed to limit an arc spread and to improve an optical output by increasing the mercury vapor pressure.
A high pressure discharge lamp is used as such a lamp, in which a pair of electrodes is arranged in an arc tube made of silica glass, so as to face each other at an interval of 2 mm or less, and in which an arc tube encloses mercury of 0.15 mg/mm3 or more, rare gas, and halogen in a range of 10−6 μmol/mm3 to 10−2 μmol/mm3, (for example, refer to Japanese Patent Application Publication No. H02-148561). Japanese Patent Application Publication No. 2009-527871 discloses such a kind of discharge lamp and a lighting device. Japanese Patent Application Publication No. 2000-131668 discloses a high pressure discharge lamp in which mercury vapor pressure of the inside of the arc tube at time of steady power lighting is 15 MPa-35 MPa, and a halogen substance in a range of 10−6 μmol/mm3 to 10−2 μmol/mm3 is enclosed in an arc tube, wherein a pair of electrodes is provided in an arc tube and a projection portion is formed around the center of the tip of the electrode, so as to prevent an arc jump phenomenon, in which the position of an electric discharge arc generated between the electrodes is not stabilized and moves around the central part or the circumference part of the tip of the electrode. Alternating current voltage is impressed between the pair of electrodes so as to light the lamp by a lighting apparatus comprising a DC/DC converter, a DC/AC inverter and a high voltage generating apparatus.
On the other hand, since projectors are getting miniaturized in recent years and are becoming home standards, it is necessary to consider a screen brightness relative to the environment in use and a projected image. A projector, which has a function called a modulated light function, is invented in order to meet such demands (for example, Japanese Patent Application Publication No. 2000-131668). Here, in the modulated light function, the brightness of a high pressure discharge lamp is adjusted so that electric power consumption is reduced by lighting the high pressure discharge lamp with electric power lower than the rated power. Hereinafter, the lamp lighting with electric power lower than the rated power is referred to as a “modulated light power lighting.” In general, existing high pressure discharge lamp lighting apparatuses have both the “rated power lighting” function and the “modulated light power lighting” function. In the present specification, lighting including “rated power lighting” and “modulated light power lighting” is defined as “steady power lighting.”
Moreover, the “modulated light power lighting” is generally operated with 60-80% of electric power in the “rated power lighting.” FIG. 5 shows an example of a waveform of current, in case where a high pressure discharge lamp is lighted by a lighting apparatus having a modulated light function. As shown in the figure, when a modulated light power lighting command signal becomes ON while the rated power lighting is carried out, the high pressure discharge lamp is lighted by electric power, which is decreased to approximately 60 to 80% of that in the rated power lighting.
In recent years, for example, a projector with a function called an “AV mute” has been also invented, in which when an image projection to a screen itself is not needed, the projection is not carried out temporarily. Since the internal pressure of the discharge lamp having such a function is high immediately after the discharge lamp is turned off, the discharge lamp cannot be re-lighted at that time. Therefore, in order to respond thereto, light to be projected on the screen is blocked by shuttering it mechanically or adjusting voltage applied to a liquid crystal panel. Lighting of a lamp in a state where an image is not intentionally projected on a screen, is defined as “low power lighting”.
It is desirable that the electric power in such low power lighting be as low as possible. By performing lighting by the utmost low power, heat generated from the lamp is remarkably reduced, so that it is possible to stop driving a cooling fan or to reduce the speed of the cooling fan, which is a principal cause of a noise from the projector, whereby the noise from the projector can be lowered to the utmost level. Furthermore, since the thermal load of the lamp can be reduced by lighting the lamp at low electric power, it is possible to substantially extend a life span of the lamp by combining the case where the image projection is required with the case where the projection is not required. The “thermal load of the lamp” means a thermal load to an arc tube and electrodes, and they can be reduced by lowering applied power.
Moreover, a contrast ratio is one of performances of a projector. The contrast ratio is a ratio of the luminance in a white state (bright state) on a projected screen and that in a state of black (dark state) on the projected screen. Since an image whose brightness and darkness are clearly shown can be projected when the contrast ratio is high, it is considered as an important performance of the projector in addition to brightness. The technology (for example, an iris function), in which a state of black is created by, for example, providing the above-mentioned mechanical shutter function in order to make a contrast ratio high, is adopted. As described above, there is a demand on a high pressure mercury lamp in which while a certain brightness or more is required in case of the rated power, the modulated light power lighting can be carried out, and further the low power lighting, in which modulated light power is extremely limited, can be carried out.
As described above, although there is a demand on a high pressure mercury lamp capable of performing low power lighting, in which electric power is extremely limited, a problem set forth below occurs, when it is used at greatly lowered electric power while a rectangle wave alternating current is supplied. Japanese Patent Application Publication No. 2006-332015 discloses that, in this kind of high pressure mercury lamp, a projection is formed at a tip portion of an electrode during lighting, whereby arc electric discharge can be stably formed, wherein the projection serves as a starting point form. According to Japanese Patent Application Publication No. 2006-332015, a projection which serves as a starting point of an arc, is maintained by changing steady frequency, and low (cycle) frequency, which is intermittently (periodically) inserted, or the number of waves, according to lamp voltage or lamp lighting electric power, so that a stable operation can be performed.
However, when a high pressure discharge lamp whose rated power was 180 W, was turned on at 90 W according to the above-mentioned technology, an arc luminescent spot was not stabilized at any combination of frequencies, so that a flicker phenomenon and a lamp “light-out” often occurred. Specifically, where electric power of 70 W or less is applied when the rated power is 180 W a flicker remarkably occurs, and, in case of 30 W or less, lamp light-out occurs.